This invention relates to a saturable reactor for use in an apparatus in which quick-rising pulses are needed such as a pulse laser apparatus.
FIGS. 17 and 18 illustrate a conventional saturable reactor disclosed in "Radio Section, Paper No. 1034 (1951)", p. 185, W. S. Melville, of which FIG. 17 is a partially cut away plan view and FIG. 18 is a sectional view of the saturable reactor. In these figures, reference numeral 1 indicates a race-track magnetic core, 2 is an electrically insulating member, 3 is a conductor winding and 4 is a cooling duct defined between a side surface of the magnetic core 1 and the insulating member 2.
The operation will now be described. In FIGS. 17 and 18, the race-track magnetic core 1 is made of a lamination of a thin magnetic tape and an insulating tape and is wound in multi-layers. The insulating member 2 is disposed around the magnetic core 1, and a conductor winding 3 is wound around the insulating member 2. Cooling of the magnetic core 1 is achieved by flowing a coolant through the duct 4 which is a gap defined between the side surface of the magnetic core 1 and the insulating member 2 and extending along the length of the magnetic core 1.
FIG. 19 is a characteristic diagram illustrating magnetic characteristics of the magnetic core used in the saturable reactor. At first, the magnetic core is at a point a or B=-Br and H=0, where it is saturated by a reverse current -Ib flowing through the control winding. As a current I flowing through the conductor winding is increased, a magnetizing force H (=2.pi. I/l ; where l is the length of the magnetic path of the core) proportional to the current I generates, whereby the magnetic material of the race-track magnetic core is magnetized in accordance with the magnetizing force H and a magnetic flux density B dependent upon the property of the magnetic material generates, and the magnetizing force substantially linearly increases through the magnetic coercive force Hc which is determined by the magnetic material and the operating frequency until it reaches a point b. At this time, the inductance L of the magnetic core is proportional to the relative permeability .mu.r=dB/dH of the magnetic core, and the voltage held by the magnetic core is V =L.multidot.(dI/dt). After the magnetic flux density B reaches point b of the saturable magnetic flux density Bs, it increases only a little even when the coercive force H is increased and the specific magnetic permeability .mu.r of the magnetic core becomes 1, the inductance L of the magnetic core is greatly decreased, so that the voltage held by the race-track magnetic core becomes substantially zero, thereby to generate a voltage across the load. Thus, the saturable reactor serves as a kind of switch in which the inductance rapidly decreases. At this time, an iron loss which is as large as twice the product of the coercive force Hc and the saturable magnetic flux density Bs which is a loss of the magnetic core generates, which causes the magnetic core to be heated. Since Hc increases as the operating frequency becomes high, a very large amount of heat is generated in the saturable reactor used in TEA-CO.sub.2 laser and excimer laser required to be switched at a high speed.
Since the magnetic core of the saturable reactor is heated due to the iron loss during operation, the magnetic core must be cooled by a cooling medium to a predetermined lower temperature.
In the conventional saturable reactor illustrated in FIGS. 17 and 18, the magnetic core 1 is cooled by a cooling medium flowing through the coolant duct 4 extending along the side surface of the magnetic core 1.
Since the conventional saturable reactor is constructed as described above, the cooling medium can remove the generated heat only from the outermost or the innermost turn of the magnetic core layer which is in contact with the cooling medium. The heat generated in the core layers located near the center of the magnetic core must be radially conducted and removed through the wound layers of the magnetic tapes and the insulating tapes, which have a very poor thermal conduction efficiency which limits the cooling efficiency of the saturable reactor to be low. Also, when the cooling duct is arranged to extend along the magnetic core, i.e., in the direction of extension of the insulating tape and the magnetic tape in the laminated layer, the total length of the flow path of the cooling medium is as long as twice the magnetic path of the magnetic core and the cross-sectional area of the flow path is relatively small. Therefore, a large pressure loss is expected when a large amount of cooling medium is flowed through the long but narrow flow path, so that a large amount of coolant cannot be used. This causes the temperature of the cooling medium to be relatively rapidly elevated as the coolant proceeds through the coolant flow path, generating an undesirable large temperature difference in the coolant at the inlet and outlet of the coolant flow path.
In order to solve the above problem, Japanese Patent Application No. 1-278501 proposes to circumferentially divide the coolant flow path along the magnetic core into two sections so that the coolant flows through each of the divided shorter sections. Even with this arrangement, the length of the coolant passage cannot be made sufficiently short, the amount of the coolant cannot be greatly increased and the temperature nonuniformity cannot be satisfactorily removed.
Particularly, in saturable reactors operated in a high frequency region such as those used in switching elements for generating quick-rising pulses, the magnetic core cannot be sufficiently cooled and the operation of the saturable reactor cannot be repeated at a high rate.
FIG. 20 is a partial cut-away perspective view of another example of a conventional saturable reactor, and reference numeral 51 indicates substantially ring-shaped magnetic cores, 52 is an inner first conductor disposed within the magnetic cores 51, and reference numeral 53 is an outer second conductor disposed around the magnetic cores 51 and connected to the first conductor 52 through a load (not shown). The first conductor 52 and the second conductor 53 together constitute a conductor winding. A control winding 54 for setting up the initial state of the magnetic core 51 is provided.
In FIG. 20, the magnetic core 51 is a substantially ring-shaped member formed by winding a thin metallic tape and a nonmagnetic tape together in a plurality of laminated layers, and the first conductor 52, the load (not shown), and the second conductor 53 are electrically connected in series in the named order to constitute a saturable reactor. The magnetic characteristic of the magnetic core used in the saturable reactor is similar to that shown and described in conjunction with FIG. 19.
The inductance of the actual saturable reactor L.sub.total when the magnetic core is un-saturated can be expressed by the sum of the inductance of the un-saturated magnetic core L.sub.unsat and the stray inductance L.sub.stray other than that in the magnetic core. That is, when the saturable core is unsaturated, EQU L.sub.total =L.sub.unsat +L.sub.stray ( 1)
and when the saturable core is saturated, EQU L.sub.total =L.sub.sat +L.sub.stray ( 2)
The characteristic of the saturable reactor as a switch is determined by the ratio of the equation (1) to the equation (2) or equation (1)/equation (2), and the larger this ratio the better the switching characteristic of the reactor.
FIGS. 21 and 22 are a diagramatic cross-sectional view and a diagramatic longitudinal-sectional view, respectively, of the saturable reactor for explaining the above, in which R1 indicates an outer diameter of the inner conductor 52, R2 is an inner diameter of the outer conductor 53, and L indicates an axial length of the saturable reactor. The cross-sectional area As of an annulus defined between the first and the second conductors 52 and 53 is (R2-R1).multidot.L. When the magnetic core saturates and .mu.r=1 and (R2-R1)&gt;R1 stand, where .mu.o is magnetic permeability of vacuum, the inductance of the saturable reactor L.sub.total can be expressed by the following equation: EQU L.sub.total =.mu.o.multidot.As/2.pi..multidot.R1 (3)
On the other hand, when the magnetic core is unsaturated, i.e., .mu.o&gt;&gt;1, the inductance of the saturable reactor L.sub.total can be expressed by the following equation: EQU L.sub.total =.mu.o.multidot..mu.r.multidot.Am/l+.mu.o(As-Am)/2.pi..multidot.R1(4)
where, Am is an effective cross-sectional area of the magnetic cores 51. Assuming that 2.pi..multidot.R1.apprxeq.l, the ratio between the equations (3) and (4), which indicates the characteristic of the saturable reactor as a switch, can be expressed by the following equation: EQU (.mu.r-1).multidot.(Am/As)+1 (5)
In the conventional saturable reactor illustrated in FIG. 20, an air gap must be provided between the magnetic cores 51 and the first and the second conductors 52 and 53 so that the control winding 54 (FIG. 20) may be disposed within the air gap, so that Am becomes much larger than As. Therefore, since Am/As=0.5 in the saturable reactor of such structure, the switching characteristics is reduced to half. Also, since the relative magnetic permeability decreases as the frequency used increases, the relative magnetic permeability .mu.r of the magnetic member decreases to several hundreds to thousands in the saturable reactor used in a pulse laser apparatus in which the frequency region of the current is very high, so that, when the stray inductance L.sub.stray other than the magnetic core becomes sufficiently small as compared to the inductance L.sub.unsat of the magnetic core, the switching characteristics become very poor.
Since an air gap must be provided between the magnetic core and the first and the second conductor for disposing the control winding therein in the conventional saturable reactor, the stray inductance caused by this air gap is large, and the switching characteristics degrade in a saturable reactor which operates in a high frequency region such as those used in a pulse laser apparatus in which pulses having a particularly quick pulse rising-time are needed.
FIG. 23 illustrates, in a sectional schematic diagram, a conventional laser oscillator disclosed in Applied Physics Letter, Vol. 48. No. 23,1574 (1986). For example, in which reference numeral 81 indicates a laser chamber, 82 and 83 are first and second main electrodes disposed in an opposing relationship within the laser chamber 81. Reference numeral R4 is a laser excitation discharge, 85 is a circuit component chamber, filled with an electrically insulating oil and disposed adjacent to the laser chamber 81, for accommodating one portion of a discharge excitation circuit for generating the laser excitation discharge 84 immersed within the insulating oil, 86 is a first insulating plate serving as a barrier wall between the laser chamber 81 and the circuit component chamber 85, 87 is an electrically conductive plate for conducting a high voltage to the second main electrode 83, 88 is a first capacitor for applying a high pulse voltage across the first and the second main electrodes 82 and 83 for initiating the excitation discharge, 89 is a first charging bank for charging the first capacitor 88, 90 is a second capacitor for supplying an excitation energy into the excitation discharge, 91 is a dielectric forming the second capacitor 90, 92 is a second bank for charging the second capacitor 90, and 93 is a magnetic saturable switch for storing the energy of the second capacitor 90.
A laser gas is filled within the laser chamber 81, and the second charging bank 92 is operated to pulse charge the second capacitor 90. Then the first charging bank 89 is operated to pulse charge the first capacitor 88. At this time, the charging voltage on the first capacitor 88 is also applied across the first and the second main electrodes 82 and 83 because the first capacitor 88 is connected in parallel to the first and the second main electrodes 82 and 83. When the voltage across the first and the second main electrodes 82 and 83 reaches its discharge initiating voltage, the electric charge stored on the first capacitor 88 is discharged across the first and the second main electrodes 82 and 83 to define a laser excitation discharge 84. By pre-ionizing the region in which excitation discharge 84 takes place beforehand with X-rays, ultra-violet rays and the like, the laser excitation discharge 84 becomes a uniform discharge suitable for the laser excitation. When the laser excitation discharge 84 is initiated, the magnetic saturable switch 93 is switched to allow the electric charge stored on the second capacitor 90 to rush into the laser excitation discharge region, whereupon the laser oscillation is initiated.
In such a laser oscillator, the laser chamber 81 is filled with a laser gas, and the circuit component chamber 85 containing one portion of the excitation circuit (electric circuit components) is filled with an electrically insulating oil in view of the requisite strong insulation. When the laser excitation discharge 84 is initiated, the magnetically saturable switch 93 is switched to allow the electric charge stored on the second capacitor 90 to flow into the laser excitation discharge portion thereby to initiate the laser oscillation.
In the above oscillator, the laser gas is filled within the laser chamber 81, and an electrically insulating oil is filled within the circuit component chamber 85 accommodating one portion of the excitation circuit (electric circuit components). Therefore, any barrier wall or partition is necessary between the laser chamber 81 and the circuit component chamber 85, and an electrically insulating plate 86 made of an insulating material is necessary because a high voltage must be supplied to the second main electrode 83. With the excimer laser in which the laser is oscillated in a ultra-violet region and the laser gas is corrosive, the insulating material to be used must be tetrafluoroethylene known as Teflon (trade name), vinyl chloride resin or the like.
With the conventional laser oscillator as above constructed, the insulating oil passes through the first insulating plate made of tetrafluoroethylene, vinyl chloride resin or the like and intrudes into the laser chamber, deteriorating the laser gas. Also, since the insulating plate is subject to the internal pressure in the laser chamber, the first insulating plate made of tetrafluoroethylene and vinyl chloride resin has difficulty maintaining its mechanical strength.